Polyvinyl chloride (PVC) is the third highest volume plastic after polyethylene and polypropylene; global demand is expected to reach 80 billion pounds by 2016. Polyvinyl chloride is produced by polymerization of the vinyl chloride monomer (VCM) which is manufactured either by the oxidative chlorination of ethylene to produce 1,2-dichloroethane which is thermally cracked to produce VCM or by the direct hydrochlorination of acetylene to produce VCM in very high selectivity in a single step. The oxychlorination method generates the majority of VCM, although this route is multi-step, non-selective, and energy intensive. Production of VCM from direct hydrochlorination of acetylene is simple and very selective (>99%), although HgCl2/C, the catalyst currently used, undergoes reduction to Hg metal after a short period of time and is volatilized into the environment. In areas such as China where coal is plentiful, the formation of acetylene from coke (CaC2+H2O reaction) is well-developed technology. Thus, it is necessary to develop more environmentally-friendly catalysts for acetylene hydrochlorination.
Carbon-supported Au catalysts have been found to be active and selective catalysts for this reaction. Recent results from our laboratory have shown that, regardless of hydrogen chloride (HCl) pretreatment conditions, all Au/C catalysts exhibit the same activities after 2-3 h of reaction time, indicating that all Au particle sizes and surface compositions are similar. Sintering of Au particles from an initial value of 2 nanometers to a final value of 20 nanometers lowers the exposed surface area of Au by 90%, leading to a concomitant loss of catalytic activity. More recent work has also shown that sintering of Au is directly linked to exposure to HCl, which is, of course one of the essential reactants. If Au can be maintained in the active AuClx state during reaction and sintering can be prevented, direct hydrochlorination becomes commercially feasible.
Supported Au catalysts show high activity and selectivity for hydrochlorination of acetylene to vinyl chloride formation. Unfortunately, in the presence of HCl (one of the two reactants), Au particles undergo rapid agglomeration from approximately 2 nm in diameter to form Au particles>20 nm in diameter. FIGS. 1A and 1B depict this agglomeration. As shown in FIG. 1A, the support 10 includes a plurality of Au nanoparticles 14 (e.g., with a diameter of about 2 nm) on its surface 12. Upon exposure to HCl, the Au particles agglomerate into a larger particle 16 (e.g., with a diameter greater than 20 nm) that defines a surface 18 of AuClx, as shown in FIG. 1B. This loss of Au surface sites greatly limits catalytic activity, making this catalyst and process much less attractive.